Acetylcholine Stimulates the Release of Nitric Oxide from Rat Spinal Cord

After approval by the Animal Care and Use Committee of our institution, adult male Sprague-Dawley rats were deeply anesthetized with 50 mg/kg intraperioneal sodium pentobarbital, decapitated, and the aorta removed. The aorta was cut into 3-4-mm long rings and endothelium denuded by rubbing with stainless steel wire, then rings were mounted on transducers and tension was measured continuously with a Grass #7 polygraph (Quincy, MA). The rings were stretched to their optimum length-tension relationship by repeated exposures to 180 mM potassium chloride. Removal of endothelium was confirmed by preconstriction with phenylephrine 10 sup -6 M and lack of relaxation to 10 sup -7 M to 10 sup -6 M acetylcholine. Two rats were then killed as stated earlier and spinal cords were removed. Each spinal cord was divided into two parts, then chopped in 0.5-mm thick slices. Tissue sections from each hemispinal cord were put into an incubation chamber surrounded by a temperature-controlled water bath maintained at 26 degrees C. Tissue slices were perfused continuously with a multichannel pump (Manostat, NY) at 4 ml/min with oxygenated modified Krebs-Henseleit solution containing indomethacin 10 sup -5 M (composition in mM, 118.3 NaCl, 4.7 KCl, 2.5 CaCl², 1.2 MgSO sub 4, 1.2 KH²PO⁴, 25 NaHCO³, 0.027 EDTA, and 11 glucose), gassed with 95% Oxygen², 5% CO²at 26 degrees C. Previous studies demonstrated this tissue preparation and temperature yielded consistent responses to stimulators of nitric oxide synthase. [12]The effluent of spinal cord tissue chambers dripped onto the rings directly. Time from spinal cord tissue exposure to contact with aortic rings was < 2 s. Experiments were started after spinal cord slices had been incubated in the chamber for 60 min.

The aortic rings were preconstricted by addition of 10 sup -6 M phenylephrine into the spinal cord perfusion solution. This resulted in an increase in tension in the aortic rings of 600-900 mg (approximately 40-50% of maximum constriction possible with exposure to higher concentrations of phenylephrine), which remained stable for 60 min in control experiments. Acetylcholine was then added to the perfusion solution, beginning at 10 sup -12 M, and increased in log units to 10 sup -4 M (n = 11 rings). These data from acetylcholine alone were used as a comparison group to all other experimental treatments. To determine the receptors acted on by acetylcholine, the nonsubtype-selective muscarinic antagonist, atropine (n = 7), or the M1 subtype-selective antagonist, pirenzepine (n = 7) or M2 subtype-selective antagonist, AFDX-116 (n = 6, all 10 sup -5 M), were added to the perfusion solution beginning 20 min before acetylcholine. To ascertain whether the vasodilation caused by spinal cord tissue perfusion with acetylcholine was from nitric oxide, the following inhibitors of nitric oxide synthase or nitric oxide action were added to the perfusion solution: N-methyl-L-arginine (n = 7; 10 sup -5 M), 7-nitroindazole (7-NI; 10 sup -5 M), methylene blue (n = 7; 10 sup -5 M), and hemoglobin (n = 8; 10 sup -5 M). Methemoglobin was removed by incubation of hemoglobin solution overnight through a semipermeable membrane with sodium nitrite. At the end of experiments, sodium nitroprusside, 10 sup -6 M, was added to perfusate to yield complete relaxation.

All compounds were obtained from Sigma Chemical (St. Louis, MO) except AFDX-116, which was donated by Boehringer-Ingleheim (Ridgefield, CT). Drugs were dissolved in Krebs solution freshly prepared for each experiment, except the stock solutions of acetylcholine and phenylephrine, which were stored at -20 degrees C. Indomethacin was dissolved in 3 x 10 sup -5 M NaHCO³solution and diluted to 10 sup -5 M final concentration in the Krebs-Henseleit solution.

All results are expressed as means+/-SEM. Statistical differences were determined by using two-way analysis of variance for repeated measures. P < 0.05 was considered significant.

Endothelium was successfully removed from the aortic rings, as evidenced by lack of relaxation response to two doses of acetylcholine (data not shown). Addition of acetylcholine to spinal cord perfusion caused concentration-dependent relaxation of the detector rings, with maximum effect at 10 sup -8 M (Figure 1). Pretreatment with the nonsubtype-selective muscarinic receptor antagonist, atropine, or the M1-selective antagonist, pirenzepine, or the M2 selective antagonist, AFDX-116, significantly blocked the acetylcholine-induced aortic ring relaxation (Figure 2). To further determine whether the relaxation caused by acetylcholine perfusion with spinal cord slices was produced by nitric oxide, the nitric oxide synthase inhibitor, N-methyl-L-arginine, the nitric oxide scavenger, hemoglobin, and the soluble guanylate cyclase and nitric oxide synthase inhibitor, methylene blue were used. Each of these agents antagonized acetylcholine perfusion-induced relaxation (Figure 3). The muscarinic antagonists, N-methyl-L-arginine, hemoglobin, and methylene blue had no effects on preconstricted aortic ring tension when administered without perfusion over spinal cord slices (data not shown). Sodium nitroprusside, added at the completion of experiments, relaxed preconstricted vascular rings by greater or equal to 95% compared to baseline tension.

The role of spinal cholinomimetic agents as adjuncts to spinal anesthesia and for postoperative analgesia is being determined in ongoing clinical trials. The current study provides evidence that cholinomimetic agents may stimulate nitric oxide synthesis in the spinal cord, and previous in vivo experiments suggest that this nitric oxide synthesis is necessary for the expression of analgesic [10]and hemodynamic [7]effects of spinally administered cholinomimetic agents.

A variety of methods has been used to measure nitric oxide synthesis in central nervous system tissue, each with its own advantages and drawbacks. [13]Direct measurement of nitric oxide is limited in most cases by the instability of the molecule itself and by sensitivity of the assay. As such, several indirect measures of nitric oxide synthesis or action have been devised. We chose not to measure nitric oxide synthesis by enzymatic analysis (such as conversion of¹⁴Carbon-arginine to¹⁴Carbon-citrulline) because we wanted to maintain some anatomic integrity rather than using homogenized tissue.

We recently modified a bioassay system for nitric oxide synthesis from vascular endothelium to investigate the control of nitric oxide synthesis in spinal cord tissue. In previous experiments with this system we demonstrated that N-methyl-D-aspartate, thought to stimulate spinal cord nitric oxide synthesis based on behavioral experiments, [14]results in a concentration-dependent release of a vasorelaxant sharing the pharmacology of nitric oxide. [12]Although the chemical identity of the relaxant cannot be determined with absolute certainty, blockade by inhibitors of nitric oxide synthesis or action provides reasonable evidence that the relaxant is nitric oxide. The current study demonstrated release of a vasorelaxant from spinal cord slices by acetylcholine, which was inhibited by an nitric oxide synthase blocker, an nitric oxide scavenger, and an inhibitor of guanylate cyclase, consistent with its identity as nitric oxide. The maximum degree of vascular relaxation from acetylcholine perfused through spinal tissue in these studies is similar to that produced using the same method and perfusion with a prototypical activator of nitric oxide synthase, N-methyl-D-aspartate. [12].

Both the nonsubtype-specific muscarinic antagonist, atropine, and the subtype selective antagonists, pirenzepine (M1) and AFDX-116 (M2), inhibited vascular relaxation from perfusion of spinal cord tissue with acetylcholine. Because a variety of antagonist concentrations was not tested, it is not possible to determine the relative potencies of each antagonist on acetylcholine's effect. Both M1 and M2 receptors have been identified by ligand binding on spinal cord dorsal horn and intermediolateral cell column, [15]and both subtypes are involved in the analgesic and hemodynamic actions of spinally administered cholinomimetic agents. [7,16]Whereas we did not perform experiments with a wide range of antagonist concentrations, the current results are consistent with these observations of neuronal actions of acetylcholine on both muscarinic receptor subtypes in the spinal cord.

The current study, using whole spinal cord slices, is unable to determine the sites of vasorelaxant release stimulated by acetylcholine. The vast majority of nitric oxide synthase in the spinal cord is of the neuronal isoform, with < 5% consisting of the membrane-bound, endothelial form. [12]Similarly, it has been suggested that 7-NI is a specific inhibitor of neuronal nitric oxide synthase, [17]and inhibition of acetylcholine-induced relaxation by this agent is consistent with an acetylcholine action on neuronal nitric oxide synthase. Nitric oxide synthase is localized to the superficial dorsal horn and the intermediolateral cell column regions of the spinal cord, [18]but the current study used tissue including both areas and was therefore unable to distinguish separate release from each area.

In summary, acetylcholine perfusion of spinal cord tissue in vitro causes concentration-dependent release of a vasorelaxant with pharmacologic properties consistent with nitric oxide. Blockade by muscarinic antagonists confirms in vivo experiments and supports the concept of nitric oxide mediation of analgesic and hemodynamic actions of spinally administered cholinomimetic agents.

The authors thank Dr. Ping Li, for help in designing and performing the bioassay experiments.